1. Field of the Invention
This invention relates to devices for protecting the input and output terminals of microwave circuits from excessive voltages. In particular, it relates to such devices having an FET switch selectively shunting the terminal to ground with a gate biasing circuit for controlling operation of the FET.
2. Description of the Prior Art
Devices for limiting the electrical energy reaching a device through one conductor or another, have existed for at least a generation. A common type of these is formed by placing a pair of silicon diodes back-to-back between the line to be protected and ground. When the voltage across the diodes rises above the breakdown threshold, the diodes conduct, shorting the excess power to ground. Some of the energy is reflected back out of the port of entry, some is absorbed in the diodes, and a small amount inevitably leaks through to the device to be protected.
If the incident high-power pulse has a fast rise time, energy can leak past the diodes before they have time to break down. This is termed spike leakage and, with silicon diodes, it can be enough to damage a GaAs MESFET. Since silicon diodes cannot be integrated into a gallium arsenide monolithic circuit, this type of protection would have to be externally connected to the chip to be protected. The additional bulk of the externally connected limiter would defeat the purpose for using GaAs MMICs in many applications such as phased-array radar and would increase the cost of the unit.
An alternate approach is to fabricate a gallium arsenide diode on the chip with the device to be protected. There are two types of semiconductor diodes. One consists of three different layers in which a thin undoped layer is sandwiched between a layer enriched with hole carriers and a layer enriched with electron carriers (the PIN diode). Also, a PN diode would work. This is the same as a PIN diode with an infinitely thin I layer. The other type consists of a film of metal evaporated directly onto a doped semiconductor. If the metal does not dissolve into the semiconductor, this forms a Schottky diode. In general the PIN diode offers a larger volume for absorbing electrical energy than does the Schottky diode. For this reason the PIN diode would be preferred; however, to achieve the speed necessary to function in the microwave regime, the carriers must be electrons and not holes. Therefore GaAs MMICs are always doped with elements that give rise to electrons as the dominant carrier species. To dope a portion of the chip with hole bearing elements in order to make a PIN structure would require additional processing steps and thus increase the cost of the product.
Shunt bipolar transistors are used to suppress transient signals from a power source in lower frequency applications, as is described in U.S. Pat. No. 4,849,845 issued to Schmitt for "Transient Suppressor". With this device, the base of the transistor is controlled by a sense and control circuit to turn the transistor switch on at a first voltage and off when the voltage reaches a level less than the first voltage. A resistor and optionally a diode are in series with the transistor. Such a system requires an elaborate control circuit, requires two transistor circuits for alternating current, and is not functional at microwave frequencies.
A more simple circuit is disclosed in U.S. Pat. No. 5,041,889 issued to Kriedt et al. for "Monolithically Integratable Transistor Circuit for Limiting Transient Positive High Voltages, such as ESD Pulses Caused by Electrostatic Discharges on Electric Conductors". This circuit uses a shunt bipolar transistor with the base biased by the parallel combination of capacitor and resistor connected to the input. A second capacitor connects the base to ground. Besides having the limitations of bipolar transistor operation, it is only functional on positive surges.
In U.S. Pat. No. 4,712,152 entitled "Semiconductor Integrated Circuit Device", Iio discloses the use of parallel NPN bipolar transistors, with bases and emitters connected to ground. The transistors have different breakdown voltages for accommodating power dissipation for different types of surges.
It is also known to use FETs in shunt between an RF input and ground. In U.S. Pat. No. 3,777,216 entitled "Avalanche Injection Input Protection Device", Armstrong discloses the use of an IGFET or MOSFET input shunt with a diode coupling the gate to ground. The IGFET is said to operate in an avalanche mode for one polarity of signal and in a conduction mode in the other polarity. This device requires avalanche breakdown of the drain-gate junction for operation and does not use active biasing.
Miyazawa et al., in U.S. Pat. No. 4,893,157 entitled "Semiconductor Device", disclose using two parallel shunt transistors separated in the signal path by a resistor. One or both of the transistors are IGFETs or MOSFETs. This device also requires the use of both N-type and P-type material, and does not use active biasing.
Sasaki discloses a similar device in U.S. Pat. No. 4,423,431 entitled "Semiconductor Integrated Circuit Device Providing a Protection Circuit". With this device, the gate of an IGFET or MOSFET is biased by a resistor in the input signal path and a capacitor connecting the input to the gate. A resistor, and optionally a parallel capacitor, couple the gate to ground. The silicon gate is said to be fabricated along with the internal circuit to be protected.
Yet another variation is disclosed by Arizumi in U.S. Pat. No. 4,527,213 entitled "Semiconductor Integrated Circuit Device with Circuits for Protecting an Input Section Against an External Surge". This variation requires a MOSFET shunt associated with the output side of each of two series input resistors. One MOSFET has a gate shorted to the drain which is grounded via a resistor. The second MOSFET is the same but without the resistor to ground. Other variations are also shown directly connecting a shunt IGFET gate to the source or leaving the drain unconnected.
Another similar device using two series shunt IGFETs is disclosed by Koike in U.S. Pat. No. 4,609,931 issued for "Input Protection MOS Semiconductor Device with Zener Breakdown Mechanism". As with the device of Sasaki, these devices all require an inline resistor and use of a special diffused region under a MOSFET.
Sitch, in U.S. Pat. No. 4,930,036, discloses an "Electrostatic Discharge Protection Circuit for an Integrated Circuit". This protection circuit uses a shunt MESFET on a signal line after a series resistor. The source and gate are separated by a second series resistor. A normally reversed-biased diode is connected between the FET gate and a low voltage source. The transistor conducts when a positive discharge is applied to the input terminal, and the diode conducts when a negative discharge is applied. This device has a gate-to-channel capacitance in the switch FET that must be charged through the in-line resistor to turn the FET on. This results in a delay in operation of the switch FET after a surge is applied. Further, the FET must be an enhancement-mode type to be off and to avoid degrading the functioning circuit when the input voltage is low.
In "High Power Control Components Using a New Monolithic FET Structure", IEEE 1989 Microwave and Millimeter-Wave Monolithic Circuits Symposium, pages 51-56, Shifrin et al. disclose three current-limiting devices that use a new monolithic switch FET design that reportedly overcomes the breakdown voltage limitation of a conventional switch FET to increase its power-handling capability. One involves a series connection of FET cells coupling the RF input to ground. The FETs are controlled by a common control signal balanced to make the FETs operate as concurrently as possible. Capacitors are applied between the FETs to compensate for parasitic capacitance. This circuit is reported as being effectively tested at 27 watts, with a similar four-cell device having a power control capability of 40 watts.
Shifrin et al. also disclose a set of FETs in parallel between various points on the input signal line and ground. There is no description of the control scheme, but it reportedly is used to control 40 watts. A third version is summarily discussed as involving a voltage-controlled attenuator with a voltage multiplier and operational amplifier providing voltage detection and feedback.
Specific integrated circuit structures to effect particular protection characteristics are disclosed in U.S. Pat. No. 4,807,080 issued to Clark for "Integrated Circuit Electrostatic Discharge Input Protection", apparently requiring the use of MOSFETs in a breakdown mode, and in U.S. Pat. No. 4,803,527 entitled "Semiconductor Integrated Circuit Device Having Semi-Insulator Substrate" and issued to Hatta et al. The device disclosed in this latter patent has a series resistor in the signal path and a depleted N layer under a floating gate or insulator, for use in conjunction with an associated electrostatic destruction protect circuit.
These FET devices are designed for operation in enhancement mode and most of them are designed to be used on digital or direct current terminals rather than microwave signal lines. There remains a need for a protection circuit using a shunt MESFET that is also operable in depletion mode, has a fast switch time in response to positive and negative power surges, and is able to give protection against large AC signals without significantly degrading the performance of the protected circuit. It is also desirable to have such a device that is self-biasing and is operable for both moderate and high overvoltage conditions, i.e., in ballast and breakdown modes. Further, such a circuit having a simple sense and control circuit is also desirable.